The invention relates generally to the field of image processing and more particularly to image processing systems which adjust the brightness characteristics of a digital image.
Many digital imaging systems include three main components: a mechanism for generating the source digital imagery, a mechanism for processing the digital image data, and a mechanism for visualizing the imagery. As such, many digital imaging systems employ more than one image processing method, or algorithm, designed to enhance the visual quality of the final rendered output. In particular, image processing methods of interest are methods for adjusting the overall balance, or brightness of digital images.
In the journal article Automatic Color Printing Techniques published in Image Technology, April/May, 1969, the authors J. Hughes and J. K. Bowker describe an automatic method of printing color negative film onto to photographic paper. In this article, Hughes et al. compare their method to the predominant method of the time, namely large area transmission density (LATD). The LATD method, which involves sensing the color of the overall film negative, is described as failing to accurately predict the color balance for natural scenes which are dominated by a single color. The LATD measurements are reliable only when the scene is composed of a random sampling of red, green and blue objects. The new method described by Hughes et al involved the steps of scanning the film negative with a red, green, and blue sensitive line scanner capable of resolving reasonable spatial detail, developing two color-difference signals by subtracting the green signal from the red signal and the blue signal from the red signal, forming a spatial derivative of the color-difference signals, calculating an average color balance for the film negative by rejecting image region not exhibiting color activity, and exposing the film negative onto photographic paper using the calculated color balance to adjust the overall color of the print. The differentiation operation employed by Hughes and Bowker involves the calculation of subtracting adjacent signal values i.e. forming a gradient signal. Hughes and Bowker identified a link between regions of images which exhibit spatial activity and the likelihood of those image regions as being good estimates of color balance.
In U.S. Pat. No. 5,016,043, Kraft et al. disclose a method of color balancing and brightness balancing for photographic optical printers. In the disclosure photographic film negative originals are scanned photoelectrically by regions and three color densities are determined for each scanning regions. Each scanning region has multiple photoelectric response values produced with a high resolution scanning system. A detail contrast parameter describing the detail contrast in the scanning region is calculated by finding the maximum and minimum values taken form the multiple photoelectric response values. The detail contrast parameters for each of the scanning regions are evaluated together with the color densities of the scanning regions for the determination of the exposure light quantities. These exposure values are used to control the amount of light passing through the photographic film negative onto photographic paper and relate to the average density of the photographic film sample. In particular, in the correction of densities, scanning regions with higher detail contrasts are considered stronger than those with lower density contrasts, while color corrections are carried out in exactly the opposite manner.
In U.S. Pat. No. 4,984,013 T. Terashita describes a method of calculating the amount of exposing light for a color photographic film negative involving the steps scanning the original negative in red, green, and blue color sensitivities photoelectrically, calculating color density difference values for the red, green and blue signals of adjacent pixel values, comparing the color density difference values to a threshold value, classifying the pixels as either belonging to subject or background regions based on color difference values, calculating a printing exposure based on a statistical quantity sampled from the subject region of pixels. Alternately, the method describes the use of a color chrominance signal for forming the color density difference values. The method described by Terashita builds on the principles described by Hughes and Bowker by extending the idea of using spatial derivatives to calculate brightness balance for printing exposure control. However, the method described by Terashita does not teach a method for adjusting the brightness of a digital image. Furthermore, the formulation of the pixel classification on the basis of color and/or chrominance signals rather than luminance signals makes the method more susceptible to noise.
All the methods described above use either the red, green, blue image channels or color channel differences as the basis signal from which to derive brightness modifications. The use of a luminance channel derived from the red, green, blue image channels or sensed directly for the purposes of deriving brightness modification information is also well known in the art and practiced for many years. In all these methods, the contribution of green light is always equal to or greater than the contribution of red light. While all these methods work well, blue image channel information does not contribute to the predictability of the brightness information to the same degree as either the red or green image channels. In addition, the red image channel can yield better predictability than the green image channel.
Object of the inventionxe2x80x94What is needed is an image signal derived from the red, green, and blue image channels which emphasizes the red channel more than the green channel and the green channel more than the blue channel to take advantage of the improved predictability of the red channel information for the purposes of deriving brightness prediction information.